Voyager-1 departs to interstellar space
It's the relentless question that Voyager-1 scientists get asked all the time: "Are we nearly there yet?"
I myself have asked it three times of Ed Stone this past year.
When I sat down with the mission's project scientist in California in August 2012, his response was much the same as always: "My best estimate is that it will be in the next couple of years, but it may be in the next couple of days. It's unknown."
Not anymore. As we chatted on that hot summer day, Voyager-1 was actually just a "couple of weeks" from crossing into interstellar space.
Data gathered by the Plasma Wave Science (PWS) instrument on Voyager has now prompted the team to announce the exit occurred on 25 August 2012.
Stone has always been very careful not to use the phrase "leave the Solar System", mindful that his spacecraft still has to pass through the Oort cloud where there are comets gravitationally bound to the Sun, albeit very loosely. But in a new domain of space, it certainly is. And it's an extraordinary moment.
Think the first humans to leave Africa, think Magellan, think Gagarin and Armstrong - it's an exploration milestone.
Voyager's epic journey
It's been interesting also to watch the process of science at work, to see how this decision to call the exit came about.
We all knew something was up in mid-2012. Voyager's instruments had started to see a drop off in the numbers of energetic particles coming up behind it from the Sun. At the same time, there was a big jump in the low-energy cosmic rays hitting the detectors from the front. These lines of evidence indicated the probe had to be right at the very boundary between the Sun's domain and the medium that is filled with stuff from other stars - the interstellar medium. But was it over the line?
Some scientists thought so. In March this year, a paper appeared in the journal Geophysical Research Letters and was accompanied by a press release from the American Geophysical Union with a headline guaranteed to be noticed: "Voyager 1 has left the solar system, sudden changes in cosmic rays indicate". The media went into overdrive, as you'd expect, but within the hour I'd got a direct Twitter message from Nasa's Jet Propulsion Lab, urging me to hang on because the GRL paper did not reflect the consensus of the mission team. Voyager had yet to see an expected switch in the magnetic field data, Stone and colleagues said in their statement. And so we all rolled back to a lower alert status.
Alex Witze produced a superb feature in Nature magazine shortly after, in which she wrote of Stone's control over the Voyager message: "It will leave when he says it does."
Fast forward to August. Another paper, this time in The Astrophysical Journal. It argued that the magnetic changes demanded by the mission team might not be that relevant. A different model could explain the observed behaviour.
Stone's response on this occasion was slightly different to the March event. There was no flat contradiction. Rather, he said the new study would become "part of the discussion among scientists" as they tried to reconcile the complex data being acquired by Voyager's particle detectors.
Part of that changed response was undoubtedly because there was another paper in review, this time from a senior investigator on the mission itself, which would essentially confirm the exit assessment. Newly acquired measurements by Voyager's PWS had seen a big jump in the density of charged and neutral particles occupying every cubic metre of space. When that was put alongside all the other data - and arguments - Stone was happy.
"The project scientist agrees with me," said Prof Don Gurnett, of the University of Iowa and the principal investigator on the PWS. "There was quite a discussion about our result, I must say, but it's pretty clear. It's very impressive."
So, Voyager is finally out. You can debate the definition of "Solar System", but I think most people will take that to mean the heliosphere - that bubble of hot gas that surrounds our star, the Sun. The probe has crossed the boundary - the heliopause - where the matter and magnetic fields derived from the Sun butt up against the material coming from other stars. Voyager has begun a new phase of exploration.
I'm excited by it and I think a lot of other people will be, too.
Chris Riley is the visiting professor of science and media at the University of Lincoln. He's also renowned for his space films. You may have caught the one he made on Voyager for the BBC last year. Chris mailed me to say: "It's worth reflecting on the fact that, for perhaps the first time in the history of our galaxy, a life form has succeeded in extending its physical presence beyond the influence of the star it emerged around.
"Such an achievement could be the greatest thing that we human beings ever accomplish, a monument to our existence which could outlive civilisation itself.
"Voyager-1's milestone should cause us all to pause and consider that this tiny spacecraft, now almost 19 billion km from Earth, represents us as a single species - and not as we more often see ourselves - divided by our ideologies, nationalities and religious beliefs."
Oxford University's Dr Allan Chapman - you'll have seen him on the BBC's Sky At Night with Sir Patrick Moore discussing the history of science and astronomy - sent me this tribute to Voyager.
"Farewell Voyager, after your magnificent work teaching us more about the other worlds of the Solar System.
"You were the latest 'upgrade' in optical and mechanical technology, which began in 1609 when first Thomas Harriot and then Galileo brought home to mankind that our ancestral five natural senses could be strengthened - indeed, unimaginably strengthened - by ingenious devices.
"Two spectacle lenses in a tube began the saga, 404 years ago. Then, once the penny had dropped, we quickly learned how to create ever greater magnifications.
"Then came photography, then spectroscopy - and then computing and imaging. And then, how to send our 'artificial organs' of seeing into the very depths of space, and still stay in touch with you.
"And now, you have more than earned your keep, and deserve the long, long rest that awaits you as you glide for evermore through realms of light and energy. Vola in Pace - 'Fly in Peace'. "